Effective fluorescent chemosensors for the detection of Zn2+ metal ion

Article abstract of DOI:10.1016/j.saa.2012.04.038

Benzimidazole derivatives synthesized from three components assembling condensation reaction behaves as a selective fluorescent sensor for Zn ²⁺ metal ion. These benzimidazole derivatives were characterized by ¹H, ¹³C NMR,

Full text of DOI:10.1016/j.saa.2012.04.038

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Effective fluorescent chemosensors for the detection of Zn2+ metal ion
⇑
J. Jayabharathi , V. Thanikachalam, K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Photo induced electron transfer
(PET) mechanism plays main role.
FT-IR spectra confirm the binding of
ligand with Zn2+ metal ion.
In the case of Pb2+, Fe2+, Co2+, Ni2+
Ca2+ and Hg2+ metal ions the
fluorescence enhancement is very
low.
,
"
Crystalline benzimidazole
derivatives (5) are triclinic crystal.
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzimidazole derivatives synthesized from three components assembling condensation reaction
behaves as a selective fluorescent sensor for Zn2+ metal ion. These benzimidazole derivatives were char-
acterized by 1H, 13C NMR, mass and elemental analysis. XRD analysis was carried out for 1-(4-methylben-
zyl)-2-p-tolyl-1H-benzo[d]imidazole. The increase in the fluorescence enhancement can be explained on
the basis of photo induced electron transfer (PET) mechanism. The blockage of the photoinduced electron
transfer process from benzimidazole ring to aryl fluorophore induced by Zn2+ co-ordination induced
emission enhancement.
Received 16 February 2012
Received in revised form 28 March 2012
Accepted 5 April 2012
Available online 3 May 2012
Keywords:
Sensing
Chemosensor
Benzimidazole
XRD
Ó 2012 Elsevier B.V. All rights reserved.
Photoinduced electron transfer
Introduction
level. The construction of fluorescent devices for sensing and
reporting of chemical events is currently of significant importance
Despite much effort in trying to unravel the role of metal ions in
biology, much remains to be learned about how metal cations are
trafficked and stored prior to their incorporation into different
metalloproteins [1]. Of particular interest is zinc, which is the sec-
ond most abundant d-block metal ion in humans [2]. Historically,
Zn2+ was detected using dithizone as a histochemical stain, which
revealed much about the spatial distribution of Zn2+ in various tis-
sues. Today, fluorescent probes have attracted considerable atten-
tion for the direct visualization of biological Zn2+ at the molecular
for both chemistry and biology [3–5]. Fluorescence proffers high
sensitivity among the many signal types available. Therefore, this
mode of signal transduction has been used widely in the detection
of a number of transition metal ions [6–8].
The imidazole ring system is one of the most important substruc-
tures found in a large number of natural products and pharmacolog-
ically active compounds [9]. In recent years, substituted imidazoles
are substantially used in ionic liquids [10] that have been given a
new approach to ‘Green Chemistry’. Triarylimidazole derivatives
have many biological activities, namely herbicidal [11], fungicidal
[12], anti-inflammatory [13] and antithrombotic activities [14]. In
addition, they are used in photography as photosensitive compound
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.038

144
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147
[15]. In continuation of our research work in developing fluorescent
sensor [16–19] herein, we report benzimidazole derivatives as sen-
sor for Zn2+ metal ion by solution spectral studies.
Experimental
Spectral measurements
The ultraviolet–visible (UV–vis) spectra of the benzimidazole
derivatives were measured in an UV–vis spectrophotometer
(Perkin Elmer Lambda 35) and corrected for background absorp-
tion due to solvent. Photoluminescence (PL) spectra were recorded
on a (Perkin Elmer LS55) fluorescence spectrometer. NMR spectra
were recorded on Bruker 400 MHz NMR spectrometer. Mass spec-
tra were recorded on a Varian Saturn 2200 GCMS spectrometer.
Fig. 2. FT-IR spectra of benzimidazole derivative (1) (solid line) and benzimidazole
derivatives bound with Zn2+ metal ion (broken line).
General procedure for the synthesis of ligands
A mixture of corresponding aldehyde (2 mmol), o-phenylenedi-
amine (1 mmol) and ammonium acetate (2.5 mmol) has been re-
fluxed at 80 °C in ethanol. The reaction was monitored by TLC
and purified by column chromatography using petroleum ether:-
ethyl acetate (9:1) as the eluent.
Zn2+ metal ion (Scheme S1). This is because of effective transfer
of electron from excited state of benzimidazole derivatives to
Zn2+ metal ion (Scheme S2).
Results and discussion
FT-IR characteristics of benzimidazole derivative–Zn2+ metal ion
Absorption characteristics of benzimidazole derivatives–Zn2+ metal
ion
The UV–visible absorption study is inadequate to throw light on
the molecular structure of the binding of benzimidazole deriva-
tives with Zn2+ metal ion. Fourier transform infrared (FT-IR) tech-
nique may add further information about the nature of
interaction between the benzimidazole derivatives with Zn2+ metal
ion. Fig. 2 presents comparison of the FT-IR spectra of bare
benzimidazole derivative (1) (solid line) and benzimidazole
derivative–Zn2+ metal ion (broken line). The spectrum of bare
benzimidazole derivative shows the >C=N stretching vibration at
1600 cmꢀ1. This band is shifted to 1629 cmꢀ1 for the benzimid-
azole derivative bound to Zn2+ metal ion. This confirms the binding
The absorption spectra of benzimidazole derivatives in presence
of Zn2+ ion at different concentration and also in their absence are
displayed in Fig. 1. The Zn2+ ion enhance the absorbance of benz-
imidazole derivatives remarkably without shifting its absorption
maximum appeared around 290 nm. This indicates that Zn2+ ion
do not modify the excitation process of the ligand. The enhanced
absorption observed around 290 nm with different concentration
of Zn2+ ion is due to binding of benzimidazole derivatives with
Fig. 1. Absorption spectra of benzimidazole derivatives (1–6) in presence and absence of Zn2+ metal ion.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147
145
Fig. 3. Emission spectra of benzimidazole derivatives (1–6) in presence and absence of Zn2+ metal ion.
Fig. 4. Bar diagram of emission intensity of benzimidazole derivatives (1–6) in presence of Pb2+, Fe2+, Co2+, Ni2+, Ca2+ and Hg2+ metal ions.
of benzimidazole derivative with Zn2+ metal ion. Similar trend was
versus 1/[Q]. This clearly shows that each ligand is bound to Zn2+
observed for all other benzimidazole derivatives (2–6).
metal ion.
Calculation of fluorescence quantum yield and overlap integral
Fluorescence enhancement by Zn2+ metal ion
According to Forster’s energy transfer theory, the energy trans-
fer efficiency is related not only to the distance between the accep-
tor and donor (r0), but also to the critical energy transfer distance
(R0). That is
The emission spectra of benzimidazole derivatives in presence
of Zn2+ metal ion at different concentration and also in their ab-
sence are displayed in Fig. 3. The Zn2+ metal ion enhance the emis-
sion of benzimidazole derivatives remarkably without shifting its
emission maximum around 350 nm. This indicates that the Zn2+
metal ion do not modify the excitation process of the ligand. The
enhanced emission at 350 nm observed with Zn2+ metal ion is
due to binding of the benzimidazole derivatives with Zn2+ metal
ion. Fluorescence enhancement arises due to the formation of
E ¼ R60=ðR60 þ r60Þ
where R0 is the critical distance when the transfer efficiency is 50%.
R60 ¼ 8:8 ꢁ 10ꢀ25K2Nꢀ4u
J
benzimidazole–Zn2+ metal ion complex. In the case of Pb2+, Fe2+
,
where K2 is the spatial orientation factor of the dipole, N is the
Co2+, Ni2+, Ca2+ and Hg2+ metal ions the fluorescence enhancement
refractive index of the medium,
u is the fluorescence quantum yield
is very low (Fig. 4). Fig. 5 shows the linear variation of 1/F0 ꢀ F
of the donor and J is the overlap integral of the fluorescence

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J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147
Fig. 5. Plot of 1/F0 ꢀ F versus 1/[Q].
emission spectrum of the donor and the absorption spectrum of the
acceptor. The value of J can be calculated by using the equation gi-
ven below:
is in accordance with the conditions of Forster’s energy transfer
theory [22] and suggests that electron transfer occurs between
the Zn2+ metal ion and benzimidazole derivatives (1–6) with high
probability [23].
Z
J ¼ FðkÞ
e
ðkÞk4dk=FðkÞdk
Photoinduced electron transfer mechanism
where F(k) is the fluorescence intensity of the donor and e(k) is mo-
lar absorptivity of the acceptor. These experimental conditions, the
calculated values of E, R0 and r0 are ꢂ0.13–0.45, ꢂ1.73–2.37 and
ꢂ2.05–2.88, respectively. The literature values of K2(=2/3) and
N(=1.3467) are used for the calculation [20,21] and the correspond-
The enhancement in fluorescence intensity of benzimidazole
derivatives on interaction with Zn2+ ion may be explained on the
basis thermodynamically favourable photoinduced electron trans-
fer mechanism (PET) [24] between benzimidazole derivatives and
Zn2+ ion. The Zn2+ ion is likely to bind to benzimidazole derivatives
via the nitrogen atoms. The process occurs due to transfer of
ing
u values for 1–6 from the present study. Obviously, the calcu-
lated value of R0 is in the range of maximal critical distance. This
Table 1
Selected bond lengths (Å), bond angles (°) and torsional angles (°) of 5.
Bond lengths (Å)
Experimental XRD (Å)
Bond angles (°)
Experimental XRD (°)
Torsional angles (°)
Experimental XRD (°)
N1A–C2A
N1A–C8A
N1A–C1A
N3A–C2A
N3A–C9A
N1B–C2B
N1B–C8B
N1B–C1B
N3B–C2B
N3B–C9B
C1A–C11A
C2A–C21A
C7A–C8A
C8A–C9A
C14A–C17A
C24A–C27A
1.3782(1.4972)
1.3833(1.4584)
1.4537(1.4700)
1.3163(1.3671)
1.3881(1.4606)
1.3795(1.4972)
1.3863(1.4584)
1.4550(1.4700)
1.3174(1.3671)
1.3866(1.4606)
1.5135(1.5400)
1.4729(1.5400)
1.3921(1.3862)
1.3999(1.4763)
1.5089(1.5400)
1.5078(1.5400)
C2A–N1A–C8A
C2A–N1A–C1A
C8A–N1A–C1A
C2A–N3A–C9A
C2B–N1B–C8B
C2B–N1B–C1B
C8B–N1B–C1B
C2B–N3B–C9B
N1A–C1A–C11A
N3A–C2A–N1A
N3A–C2A–C21A
N1A–C2A–C21A
N1A–C8A–C3A
N1A–C8A–C9A
N3A–C9A–C4A
N3A–C9A–C8A
106.23(101.69)
128.33(113.71)
124.79(113.33)
104.77(105.49)
106.27(101.69)
129.32(113.71)
123.98(113.33)
105.12(105.41)
115.07(109.47)
113.25(113.53)
123.44(123.23)
123.30(123.24)
131.76(130.72)
103.49(108.25)
129.91(130.69)
110.26(108.25)
C2A–N1A–C1A–C11A
C8A–N1A–C1A–C11A
C9A–N3A–C2A–N1A
C9A–N3A–C2A–C21A
C8A–N1A–C2A–N3A
C1A–N1A–C2A–N3A
C8A–N1A–C2A–C21A
C1A–N1A–C2A–C21A
C9A–C4A–C5A–C6A
C2A–N1A–C8A–C7A
C1A–N1A–C8A–C7A
C2A–N1A–C8A–C9A
C1A–N1A–C8A–C9A
C6A–C7A–C8A–N1A
C2B–N1B–C1B–C11B
C8B–N1B–C1B–C11B
C9B–N3B–C2B–N1B
C9B–N3B–C2B–C21B
C8B–N1B–C2B–N3B
C1B–N1B–C2B–N3B
C8B–N1B–C2B–C21B
C1B–N1B–C2B–C21B
C9B–C4B–C5B–C6B
C2B–N1B–C8B–C7B
C1B–N1B–C8B–C7B
C2B–N1B–C8B–C9B
C1B–N1B–C8B–C9B
C6B–C7B–C8B–N1B
109.45(ꢀ177.75)
ꢀ81.12(ꢀ62.25)
ꢀ0.02(ꢀ13.69)
ꢀ178.71(165.91)
0.59(17.54)
171.55(139.72)
179.28(ꢀ162.06)
ꢀ9.75(ꢀ39.88)
0.1(ꢀ0.1581)
178.45(166.15)
7.08(43.71)
ꢀ0.88(13.70)
ꢀ172.25(136.15)
179.75(176.19)
108.10(ꢀ177.75)
ꢀ79.66(ꢀ62.25)
ꢀ0.86(ꢀ13.69)
177.35(165.91)
1.24(17.54)
173.85(139.72)
ꢀ176.90(162.06)
ꢀ4.29(ꢀ39.88)
0.25(ꢀ0.1581)
176.39(166.15)
3.28(43.71)
ꢀ1.05(ꢀ13.70)
ꢀ174.15(ꢀ136.15)
177.76(176.19)
Values within the parenthesis corresponds to theoretical values.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147
147
electron density from lone pair of electrons of the nitrogen atom to
Acknowledgment
the LUMO of the benzimidazole derivatives. Binding of Zn2+ ion
with benzimidazole derivatives through the nitrogen atom lone
pairs will obviously hindered the PET process leading to enhance-
ment of fluorescence of benzimidazole derivatives on interaction
with Zn2+ ion.
One of the authors Prof. J. Jayabharathi is thankful to Depart-
ment of Science and Technology [No. SR/S1/IC-73/2010], University
Grants commission (F. No. 36-21/2008 (SR)) and Defence Research
and Development Organisation (DRDO) (NRB-213/MAT/10-11) for
providing funds to this research study.
For a PET chemosensor, a fluorophore is usually connected to a
receptor containing a relatively high-energy non-bonding electron
pair, such as nitrogen atom, which can transfer an electron to the
excited fluorophore and as a result quench the fluorescence. When
this electron pair is bound by coordination of a cation, the redox
potential of the receptor is raised so that the HOMO of the receptor
becomes lower in energy than that of the fluorophore. Thus, the
PET process from the receptor to the fluorophore is blocked and
the fluorescence is switched on. Most of the PET type chemosen-
sors sense Zn2+ with a fluorescence enhancement signal.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.038.
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ꢀ
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(Fig. S6) shows that the benzimidazole ring is essentially planar.
The dihedral angles between the planes of the benzimidazole and
the benzene rings of the 4-methylbenzyl and the p-tolyl groups
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We have developed a benzimidazole based fluorescent sensors
that showed preferential binding for Zn2+ over Pb2+, Fe2+, Co2+
,
Ni2+, Ca2+ and Hg2+ metal ions. It is excited around 290 nm and
emits around 350 nm with fluorescence enhancement after and
before complexation with Zn2+ metal ion. The high selectivity of
Zn2+ is marked by a significant fluorescent enhancement. The
enhancement in fluorescence of benzimidazole derivatives on
binding with Zn2+ ion via nitrogen atom is due to photo induced
electron transfer process.